222
7 Structuring Possibilities
with the temperature-controlled pick-and-lift technique to form the desired vertical
structure. Here, an oxygen plasma treatment of the substrate surface for WSe 2 before
exfoliation helps to reduce contamination of the flake.
Owing to the sufficient contrast under a light microscope, monolayers and thin
hBN can be both well identified by optical contrast. Typically, a dry stacking technique is then used with polypropylene carbonate (PPC) on PDMS to pickup and
stack h-BN/TMDC layers.
Interestingly, the preferred way of heterostructuring is based on consecutive
picking-up of the identified flakes resulting in the desired sequence on the polymerbased viscoelastic stamp. For the above examples [55–57], firstly, a top layer of hBN
was picked up at 48
◦ C, then WSe 2 , and finally the bottom layer of hBN. To re-smooth
the PPC and ensure a clean wave-front after each hBN pick-up step, the PPC was
briefly heated to 90
◦ C. For the final transfer of the stack onto a clean substrate, the
substrate was heated to 75
◦ C at first, and once the stack had been put into contact, it
was gradually heated to 120
◦ C. Consecutively, the PPC/PDMS was lifted. To remove
polymer residue, the substrate with stack on top was immersed in chloroform and
rinsed with isopropyl alcohol (IPA). This procedure follows the method used by the
group of J. Hone at the Columbia University [19].
While simple viscoelastic stamping of flakes can be performed with only one
polymer component as a sticky film, the combination of two films allows one to
modify the stickiness of the different polymer layers of the joint film by means
of heating, owing to the different glass temperatures of each of the used polymer
materials.
In contrast to exfoliated samples, CVD growth can directly deliver epitaxially
stacked homo- and heterolayers of 2D materials. While current work is primarily
focusing on both lateral and vertical heterostructure fabrication [12, 48], the deterministic growth of bilayers with different stacking angle is equally relevant, being
motivated by recent observations of a superconducting phase [58, 59] and other
interesting phenomena [60, 61]. In that specific superconductivity study on bilayer
graphene by Cao et al., graphene from the same flake was merely picked up by
mechanical break off and transferred with a given twist angle on top of its substratebound remainder, to give an example.
Bilayers of CVD-grown TMDCs with different symmetries such as AA’ and ABtype homobilayers of WS 2 were readily obtained after a two-step growth process, as
described in the previous section. Yet, a transfer step is typically applied to provide
the samples on the desired substrate for optical studies [14], which is as an example briefly outlined here. By covering the sample with poly(methyl methacrylate)
(PMMA) by a dropper and letting it dry, a subsequent dive in 30% KOH (aq) for a
few minutes left behind a floating PMMA/bilayer stack separated from the growth
substrate. The KOH residue was then removed from the PMMA/bilayer stack during a repeated cleaning procedure in deionized (DI) water. Afterwards, the cleaned
PMMA/TMDC sheet was attached to the target substrate, which could be for instance
SiO 2 , sapphire or hBN, dried in ambient conditions for 1 hour and later backed
at 90
◦ C for 1.5 min in order to enhance the bonding between the substrate and the
bilayers. Following this transfer procedure, PMMA was removed using 50
◦ C warm
7 Structuring Possibilities
with the temperature-controlled pick-and-lift technique to form the desired vertical
structure. Here, an oxygen plasma treatment of the substrate surface for WSe 2 before
exfoliation helps to reduce contamination of the flake.
Owing to the sufficient contrast under a light microscope, monolayers and thin
hBN can be both well identified by optical contrast. Typically, a dry stacking technique is then used with polypropylene carbonate (PPC) on PDMS to pickup and
stack h-BN/TMDC layers.
Interestingly, the preferred way of heterostructuring is based on consecutive
picking-up of the identified flakes resulting in the desired sequence on the polymerbased viscoelastic stamp. For the above examples [55–57], firstly, a top layer of hBN
was picked up at 48
◦ C, then WSe 2 , and finally the bottom layer of hBN. To re-smooth
the PPC and ensure a clean wave-front after each hBN pick-up step, the PPC was
briefly heated to 90
◦ C. For the final transfer of the stack onto a clean substrate, the
substrate was heated to 75
◦ C at first, and once the stack had been put into contact, it
was gradually heated to 120
◦ C. Consecutively, the PPC/PDMS was lifted. To remove
polymer residue, the substrate with stack on top was immersed in chloroform and
rinsed with isopropyl alcohol (IPA). This procedure follows the method used by the
group of J. Hone at the Columbia University [19].
While simple viscoelastic stamping of flakes can be performed with only one
polymer component as a sticky film, the combination of two films allows one to
modify the stickiness of the different polymer layers of the joint film by means
of heating, owing to the different glass temperatures of each of the used polymer
materials.
In contrast to exfoliated samples, CVD growth can directly deliver epitaxially
stacked homo- and heterolayers of 2D materials. While current work is primarily
focusing on both lateral and vertical heterostructure fabrication [12, 48], the deterministic growth of bilayers with different stacking angle is equally relevant, being
motivated by recent observations of a superconducting phase [58, 59] and other
interesting phenomena [60, 61]. In that specific superconductivity study on bilayer
graphene by Cao et al., graphene from the same flake was merely picked up by
mechanical break off and transferred with a given twist angle on top of its substratebound remainder, to give an example.
Bilayers of CVD-grown TMDCs with different symmetries such as AA’ and ABtype homobilayers of WS 2 were readily obtained after a two-step growth process, as
described in the previous section. Yet, a transfer step is typically applied to provide
the samples on the desired substrate for optical studies [14], which is as an example briefly outlined here. By covering the sample with poly(methyl methacrylate)
(PMMA) by a dropper and letting it dry, a subsequent dive in 30% KOH (aq) for a
few minutes left behind a floating PMMA/bilayer stack separated from the growth
substrate. The KOH residue was then removed from the PMMA/bilayer stack during a repeated cleaning procedure in deionized (DI) water. Afterwards, the cleaned
PMMA/TMDC sheet was attached to the target substrate, which could be for instance
SiO 2 , sapphire or hBN, dried in ambient conditions for 1 hour and later backed
at 90
◦ C for 1.5 min in order to enhance the bonding between the substrate and the
bilayers. Following this transfer procedure, PMMA was removed using 50
◦ C warm